Changing a C# Delegate's Calling Convention to Cdecl

Changing a C# delegate's calling convention to CDECL

By default the p/invoke system wraps your delegate in a stdcall function. You can change the generated wrapper's calling convention by using the UnmanagedFunctionPointer attribute:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void MyDelegate();

C# delegate callback with params cause AccessViolation when calling from C++ DLL

I've not looked at every detail of your code, but some commonly seen p/invoke issues just out immediately:

1. You cannot use C++ strings as interop types. You should use null terminated character arrays on the C++ side. Obtain these using the c_str() method of std::string.
2. The delegate likely has the wrong calling convention. The unmanaged code (probably) uses cdecl and so you need to use the [UnmanagedFunctionPointer(CallingConvention.Cdecl)] attribute when declaring the delegate type in C#.
3. Your C# delegate is susceptible to being collected early (while your unmanaged code still holds a reference to it) because there are no managed references to it. Typically that would be solved by holding a reference to the delegate in a static variable in your C# code.

Changing Calling convention from cdecl to stdcall

It looks like GCBOpen() is compiled __cdecl but its declaration does not explicitly state that. (That's why it linked OK when your default was __cdecl but breaks when you change it.) In general it is good practice for declarations of functions in external libraries to specify the calling convention to avoid problems such as the one you have enountered.

Somewhere you must have something like:

__declspec(dllimport)
extern int GCBOpen(int, int);

which would be better off as:

#define CALLCONV __cdecl
__declspec(dllimport)
extern int CALLCONV GCBOpen(int, int);

Unexpected calling convention for PInvoke

The header file does not specify the calling convention at all. So now you depend on the compiler default. It is configured in the MSVC++ IDE with Project > Properties > C/C++ > Advanced > "Calling Convention" setting. Default is /Gd as you'd expect, seeing it changed to /Gz is not terribly unusual as a Q&D fix.

Be careful changing it, you might break other programs that depend on stdcall in their interop code. Note that you previously asked about the pascal convention, that is stdcall in 32-bit code. Being explicit in the header file so you don't depend on the compiler default won't hurt either.

If you don't have source code then you there is no way to change it. Nothing particularly wrong with stdcall, it is the expected interop default on Windows.

calling convention when using 64bit unmanaged callback

You don't need to do anything. Calling convention directives are ignored when targeting 64 bit code because there is a single calling convention for that architecture. Leave the code as it is. It will work correctly for both 32 and 64 bit compilation.

How to pass function pointer from C# to a C++ Dll?

Your delegate uses the cdecl calling convention. In C# you would therefore declare the delegate like this:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate double CallbackDelegate(double x);

As an alternative, you could decide to declare the function pointer in C++ as __stdcall, in which case you would remove the UnmanagedFunctionPointer attribute and rely on the default calling convention being CallingConvention.StdCall.

Implement it like this:

public static double MyFunc(double x)
{
    return Math.Sqrt(x);
}

In order to keep the unmanaged function pointer alive (guarding against GC), you need to hold an instance of the delegate in a variable.

private static CallbackDelegate delegateInstance;
....
delegateInstance = MyFunc;

In the simple example that you have here, the C++ code does not use the unmanaged function pointer outside of TestDelegate, but in a more complex example you may do so, in which case you must keep the unmanaged function pointer alive.

The function that you import is declared like this:

[DllImport("DllName.dll")]
public extern static double TestDelegate(CallbackDelegate f);

You can then call it like this:

double retval = TestDelegate(delegateInstance);

Stuck on calling convention calling Managed CLI method from unmanaged C++

That is supported. You can use an attribute to tell the CLR that the thunk it creates for the delegate should use the __cdecl calling convention. Make it look like this:

using namespace System::Runtime::InteropServices;

[UnmanagedFunctionPointer(CallingConvention::Cdecl)]
delegate void OxpOnReceivedMeasurement(std::vector<OBX> *obxes);

What is the default calling convention for exported functions (VS2008)?

The default calling convention is cdecl. Note that __declspec(dllexport) has no influence on calling convention.

The calling convention can be specified in code, or by a compiler option. I don't recommend using the compiler option, it's a bit too obscure. Make it explicit in code and then anyone reading if knows what convention is used.

Note that for 64 bit Windows code, all calling conventions are equivalent which could explain what you see.

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